The present invention relates to a sample holder for mass analysis and, particularly, to a sample holder used in a secondary ion mass spectrometer for analyzing organic substances.
Determination of the molecular weight and molecular structure of various substances related to biological material is a great topic in the life and health science; theme of modern technology, and it is supported by the advancement of means for mass analyzing secondary ions produced by fast particle bombardment. In this method, a sample mixed with a large amount of matrix such as glycerol is applied to a metallic plate, and it is introduced to the ion bombardment area in a low pressure (10.sup.-5 -10.sup.-6 Torr) with a direct inlet probe. The sample with matrix molecule is sputtered by the bombardment of fast ions or neutrals having energy of 5-10 keV. Through this process, part of the sputtered material is ejected as positive or negative ions, called secondary ions", which include molecular ions of the sample. The method of mass analysis for these ions is "Termed Molecular Secondary Ion Mass Spectrometry (Molecular SIMS)".
In this method, a sample mixed with a fluid matrix such as glycerol is applied to a flat metallic plate, and introduced to the ion bombardment area. Although a fluid matrix including glycerol has a low vapor pressure, it evaporates at a considerable speed in the vacuum. Therefore, when a long term measurement is carried out, the interior of the mass spectrometer is contaminated by the fluid matrix, resulting in a degraded mass resolution and lowered sensitivity.
If the quantity of the fluid matrix is reduced significantly so as to prevent the above problems, the measurement takes a long time and the repeat accuracy of the spectrum output is impaired.
The situation will be described in more detail in connection with FIGS. 1 and 2 showing the conventional sample holder and slit plate. In this case, a sample holder 1 dimensioned by 2 mm by 5 mm holds the matrix 3 on its entire surface, providing a large matrix area in contact with the gas phase. Primary ions 2 are irradiated onto part of the matrix 3, and secondary ions 5 are released from this portion. As shown in FIGS. 1 and 2, part of the secondary ions 5 goes through the slit and reaches the ion collector. In this method, however, the matrix 3 has a large area in contact with the gas phase, and therefore a large amount of glycerol vapor, causing the contamination of the spectrometer. This problem can be alleviated by making the sample holder 1 smaller (down to about 2 mm in diameter), but at the sacrifice of the amount of sample applied, and furthermore at the risk that the matrix 3 including the sample flows over the side section of the holder 1, resulting in an increased area of matrix in contact with the gas phase.
In the usual magnetic sector type mass spectrometer, secondary ions going through the slit and reaching the ion collector are those produced by sputtering in an area of about 0.5 mm by 5 mm. Namely, only a small part of sputtered ions can be used effectively. Accordingly, a sample holder with the structure providing a minimum-necessary ion bombardment area and feeding a necessary amount of matrix has been desired.